Unidirectional transducer etched with surface acoustic waves

ABSTRACT

A surface acoustic wave transducer in which an etching network is superimposed on the conventional networks of electrodes of acoustic transducers. The superposition of these networks makes it possible to obtain a favored direction of propagation of the acoustic waves while maintaining a high quality factor Q comparable with that of conventional bidirectional transducers. Such a surface acoustic wave transducer may find application in mobile radio communication systems.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The field of the invention is that of surface acoustic wave transducersand filters including such transducers, used in numerous fields such asmobile radio communication systems for example for intermediatefrequency filtering.

2. Discussion of the Background

Various types of high-performance transducers have come out over thelast ten years or so.

Unidirectional transducers (Single-Phase Unidirectional Transducers alsoknown as SPUDT) have replaced bidirectional transducers in manyapplications by virtue of the decrease in losses which they make itpossible to obtain. This type of transducer, described in the publishedU.S. Pat. No. 2,702,899 is made by insetting, into a transducer,so-called transduction cells and so-called reflection cells, and bypositioning the cells with respect to one another in such a way that thewaves emitted are again in phase with the waves reflected in the usefuldirection and that phase opposition is obtained in the other direction.This involves a transducer in which electrodes designed so as to achievethe existence of a transduction function and a reflection function aredispersed. It has also been demonstrated in the published U.S. Pat. No.2,702,899 that it may be advantageous to make resonant cavities insidethe SPUDT, a resonant cavity being made by changing the sign of thereflection function.

For the customary substrates, the distance between transduction centreand reflection centre can be of the form (2n+1)λ/8 with n an integer sothat the phases are correct.

However, the quality factor Q relating to the ratio of the capacitanceto the conductance of the filter and representative of the bandwidth andinsertion loss of the filter is lower for unidirectional filters withtheir specific architectures into which are introduced asymmetries thanthat of conventional, symmetric bidirectional filters.

SUMMARY OF THE INVENTION

To increase performance, that is to say to increase the coupling in aunidirectional transducer (without increasing the capacitance), theinvention proposes a transducer into which is introduced an etchingnetwork superimposed on the conventional networks of electrodes ofacoustic transducers. More precisely, the subject of the invention is asurface wave transducer including a substrate on which are deposited twonetworks of interdigital electrodes and connected to differentpolarities so as to create acoustic transduction cells defined by atleast two consecutive electrodes of different polarities, characterizedin that it furthermore comprises at least one network of etchingsseparated by mesas, the superposition of the networks of electrodes andof the etching network making it possible to obtain a favoured directionof propagation of the acoustic waves.

According to a first variant of the invention, the networks ofelectrodes are symmetric with respect to an axis situated at the centreof two consecutive electrodes of the same polarity, the network ofetchings being asymmetric with respect to the said axis.

In this configuration, the symmetric networks of electrodes make itpossible to retain a high coupling factor whilst for its part theasymmetric etching network makes it possible to create the favoureddirection of propagation of the acoustic waves.

Advantageously, the surface wave transducer may comprise a succession ofat least two etching networks which are asymmetric so as to locallyreverse the favoured direction of propagation of the surface waves.

According to another variant of the invention, the networks ofelectrodes define sets of 3 electrodes of different width percharacteristic wavelength corresponding to the central frequency ofoperation of the transducer in which sets, a first electrode and asecond electrode are separated by a distance 3λ/16, and are connected todifferent polarities, the second electrode and a third electrode beingseparated by a distance λ/8, in such a way as to define a favoureddirection of propagation of the acoustic waves, the electrodes beingpositioned on the mesas of the etching network.

The advantage of such a configuration resides in the increase in thereflection coefficients of the electrodes. In conventional structures,to increase these coefficients, it is necessary to increase thethickness of the electrodes; by using a structure of mesas separated byetchings, the electrodes being deposited on the surface of the mesas,the coefficient of reflection of the electrodes is thereby increasedwhilst maintaining a small thickness of metallization.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood and other advantages will becomeapparent on reading the description which follows given by way ofnon-limiting example and by virtue of the appended figures in which:

FIG. 1a illustrates a first exemplary transducer according to theinvention with 4 electrodes per λ, exhibiting a favoured direction ofpropagation of the surface waves;

FIG. 1b illustrates a second exemplary transducer according to theinvention with 4 electrodes per λ, exhibiting a favoured direction ofpropagation of the surface waves which is opposite to that of the firstexemplary transducer;

FIG. 2 illustrates a third exemplary transducer according to theinvention with 4 electrodes per λ comprising locally a first directivityof the surface waves in one sense and locally a second directivity ofthe surface waves which is opposite to the first;

FIG. 3 illustrates a fourth exemplary transducer according to theinvention with 4 electrodes per λ in which the phase shift betweentransduction centre and reflection centre is equal to λ/16;

FIG. 4 illustrates a fifth exemplary transducer according to theinvention with 4 electrodes per λ in which the mesas and the etchings donot possess the same width;

FIG. 5a illustrates a transducer with 2 electrodes per λ according tothe prior art;

FIG. 5b illustrates a first exemplary transducer according to theinvention with 2 electrodes per λ , in which the electrodes arepositioned astride etching flanks, of an etching network of period λ2/;

FIGS. 6a and 6 b illustrate a second exemplary transducer according tothe invention with 2 electrodes per λ, in which the electrodes arepositioned on mesas or etchings of an etching network of period λ;

FIG. 7a illustrates an exemplary conventional transducer of the SPUDTtype with 3 electrodes per λ;

FIG. 7b illustrates an exemplary transducer of the SPUDT type with 3electrodes per λ, in which the network of electrodes is superimposed onthe etching network;

FIG. 7c illustrates a variant of the exemplary unidirectional transducerillustrated in FIG. 7b.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In general, the surface wave transducer according to the inventioncomprises the superposition of networks of electrodes and of at leastone etching network. Conventionally, the substrates used can be inparticular quartz, the electrodes may be obtained by metallization forexample using aluminium. The substrates used may again advantageously beof the LiNbO₃, LiTaO₃ or else Li₂B₄O₇ type. Furthermore, etchingtechniques are highly developed on such quartz type substrates and inparticular the so-called ICP (Inductive Coupled Plasma) technique usinga high-energy plasma and allowing low-cost mass fabrication of etcheddevices. It should be noted that the width of the etchings can bedifferent from the width of the mesas and in particular smaller,according to certain variants, the etching width being equal to theelectrode width.

According to a first variant of the invention, to obtain a largecoupling coefficient conferred by symmetric networks of electrodes, afavoured direction of propagation of the surface waves is created byvirtue of the presence of the etching network. We shall describe severalpossible exemplary layouts for implementing this type of configuration.

Examples of Transducers with 4 Electrodes Per Wavelength λ

In this type of transducer, the electrodes are distributed symmetricallyover the substrate with a period λ/4. Such a structure exhibits a goodcoupling coefficient but remains bidirectional. Specifically, thereflections created by the electrodes cancel one another out and nofavoured direction of propagation of the acoustic waves is generatedwith such a configuration. To disturb this bidirectionality, theinvention proposes to superimpose an etching network so as to give riseto additional reflections in an asymmetric manner with respect to acentral axis C defined between two electrodes connected to differentpolarities and symbolized in the diagram by a + sign and a − sign, inFIG. 1a.

By making the centres of the electrodes coincide with the centres of themesas or of the etchings, reflections from centre of etching positionedat λ/8 or 3λ/8, with respect to the centre of an electrode, are obtainedif the distance between consecutive mesa and etching is λ/4. Theseetching flanks generate the reflections required for obtaining afavoured direction of propagation of the surface acoustic waves. FIG. 1billustrates a configuration in which the favoured direction ofpropagation of the acoustic waves is opposite to that of FIG. 1a. Inthis first variant, the distance between consecutive electrodes mayadvantageously be equal to the distance between a mesa and a consecutiveetching.

As has been stated previously, it may be beneficial to create resonantcavities. To make this type of configuration, in which the favoureddirection of propagation of the acoustic waves is locally reversed, thetransducer according to the invention can comprise a succession oflayout according to FIG. 1a and of layout according to FIG. 1b, asillustrated in FIG. 2. In this type of layout at the level of the axisAA′, there is a break in the periodicity of the etching network such asto pass continuously from an etching network of first type R_(i) asrepresented in FIG. 1a to an etching network of second type R_(i+1), asrepresented in FIG. 1b.

Such a configuration has the benefit of much easier technology than thatconventionally used in this type of transducer for which it is tricky tolocally displace the position between a transduction centre and areflection centre so as to obtain the desired reversal of directivity.

In the transducer configurations described above with 4 electrodes perwavelength λ, a transduction centre situated at the centre of anelectrode (for example referenced +) is separated from an etching flankcorresponding to a centre of reflection by a distance λ/8, correspondingto the ideal case. In certain applications and in view of the substratesemployed, it may be beneficial to create a phase shift which differsfrom 45° (corresponding to λ/8). To do this and according to theinvention, the etching network can advantageously be shifted withrespect to the network of electrodes. FIG. 3 illustrates a configurationin which a transduction centre is a distance of λ/16 away from anetching flank, i.e. a phase shift of 22.5°. In this configuration, theelectrodes are aligned with the flanks of etchings, this possiblyrepresenting a technological facility.

Furthermore, in the aforesaid examples, the mesas and etchings exhibitthe same widths, however the latter may also advantageously be of adifferent width.

Typically, the etchings can possess a width equal to λ/8 whilst themesas possess a width equal to 3λ/8. This allows the etchings to becompletely filled with metallization as illustrated by FIG. 4. Theproduction sensitivity may thus be enhanced. Moreover, with thislatitude, it becomes very simple to change the width or the position ofthe etchings locally in the transducer, so as to modify the phase andthe amplitude of the reflection coefficient.

Examples of Transducers with Two Electrodes Per Wavelength λ

The examples described above all relate to transducers of 4 electrodesper λ, in which the favoured directivity of the surface waves is easilyobtained. According to the prior art, the transducers with twoelectrodes per λ, illustrated in FIG. 5a, exhibit very high couplingcoefficients which are greater than in the transducers with 4 electrodesper λ but are not nevertheless bidirectional, the reflections betweenelectrodes being in-phase and being so symmetrically. The superpositionof an etching network in this type of transducer advantageously makes itpossible to alleviate this drawback. Moreover, this type of transducerhas a technological advantage since it makes it possible to fabricatenetworks of electrodes with a spacing twice as large as the spacingnecessary in transducers with 4 electrodes per λ.

Conventionally, the transducers with 2 electrodes per λ, compriseelectrodes of width λ/4 separated by a spacing λ/2, as illustrated inFIG. 5a. The waves emitted at the level of an electrode E_(i) are inphase with the waves reflected by the consecutive electrode E_(i+1) andvice versa for the waves emitted at E_(i+1) and those reflected by theelectrode E_(i). To disturb this symmetry and the placing in phase ofthe reflections at the electrodes, the invention proposes to positionthe electrodes on the etching flanks as illustrated in FIG. 5b.

To comprehend the manner of operation of such a structure and how it ispossible to optimize this type of layout, we shall consider thereflection coefficients relating respectively to an etching flank, to anelectrode and sited at the centre of the electrode.

In the upward direction etching→mesa, the reflection coefficient isreferenced rg.

In the downward direction mesa→etching, the reflection coefficient isreferenced −rg*.

In the upward direction substrate→electrode, the reflection coefficientis referenced −re*.

In the downward direction electrode→substrate, the reflectioncoefficient is referenced re.

If the centre of the electrode is displaced by distance d with respectto the etching flank, the reflection coefficient Γ sited at the centreof the electrode given by the following formula $\begin{matrix}{\Gamma = {{r_{g} \cdot ^{2 \cdot j \cdot k \cdot {({\frac{\lambda}{4} - d})}}} - {\overset{\_}{r_{g}} \cdot ^{{- 2} \cdot j \cdot k \cdot d}} + {r_{e} \cdot ^{2 \cdot j \cdot k \cdot \frac{\lambda}{8}}} - {\overset{\_}{r_{e}} \cdot ^{{- 2} \cdot j \cdot k \cdot \frac{\lambda}{8}}}}} \\{= {{{- \left( {r_{g} + \overset{\_}{r_{g}}} \right)} \cdot ^{{- j} \cdot 4 \cdot \pi \cdot \frac{d}{\lambda}}} + {j \cdot \left( {r_{e} + \overset{\_}{r_{e}}} \right)}}} \\{= {{{- 2} \cdot {{Re}\left( r_{g} \right)} \cdot ^{{- j} \cdot 4 \cdot \pi \cdot \frac{d}{\lambda}}} + {j \cdot 2 \cdot {{Re}\left( r_{e} \right)}}}}\end{matrix}$

To obtain unidirectional transduction, we require the reflectioncoefficient Γ to be a pure real, thus implying: $\begin{matrix}{{{{- 2} \cdot {{Re}\left( r_{g} \right)} \cdot ^{{- j}{\cdot 4 \cdot \pi \cdot \frac{d}{\lambda}}}} + {j \cdot 2 \cdot {{Re}\left( r_{e} \right)}}} = {0\quad {where}\quad {Re}\quad {real}\quad {part}}} & (1) \\{d = {{{- \frac{\lambda}{4 \cdot \pi}} \cdot a}\quad {\sin \left( \frac{{Re}\left( r_{e} \right)}{{Re}\left( r_{g} \right)} \right)}}} & \quad \\{\Gamma = {{- 2} \cdot {{Re}\left( r_{g} \right)} \cdot {\cos \left( {4 \cdot \pi \cdot \frac{d}{\lambda}} \right)}}} & (2) \\{hence} & \quad \\{\Gamma = {{- 2} \cdot {{Re}\left( r_{g} \right)} \cdot {\cos \left( {{- a}\quad {\sin \left( \frac{{Re}\left( r_{e} \right)}{{Re}\left( r_{g} \right)} \right)}} \right)}}} & \quad \\{\Gamma = {{- 2} \cdot {{Re}\left( r_{g} \right)} \cdot \sqrt{1 - \frac{{{Re}\left( r_{e} \right)}^{2}}{{{Re}\left( r_{g} \right)}^{2}}}}} & \quad\end{matrix}$

Equations (1) and (2) impose the condition |Re(re)|<|Re(rg)|, which canalways be obtained for a judiciously chosen value of the distance d.

The first exemplary transducer with 2 electrodes per λ, which has justbeen described nevertheless requires a technology in which theelectrodes must be deposited at the intersection of a mesa and anetching, which is not very easy.

This is why, the invention also proposes another configuration oftransducer with 2 electrodes per λ, but of more direct technology.

This involves a unidirectional transducer in which the network of 2electrodes per λ is superimposed on a network of etchings of spacing λ,as illustrated in FIG. 6a or 6 b.

This type of transducer operates at the second harmonics. It has theadvantage of offering a wider geometry than the geometries describedabove and is especially advantageous for applications at very highfrequencies.

By considering the same parameters rg, re and Γ, we obtain for thecentral reflection coefficient Γ: $\begin{matrix}{\Gamma = \quad {{r_{g} \cdot ^{2 \cdot j \cdot k \cdot {({\frac{3 \cdot \lambda}{8} - d})}}} - {\overset{\_}{r_{g} \cdot}^{{- 2} \cdot j \cdot k \cdot {({\frac{\lambda}{8} + d})}}} + {r_{e} \cdot ^{2 \cdot j \cdot k \cdot \frac{\lambda}{8}}} - {\overset{\_}{r_{e} \cdot}^{{- 2} \cdot j \cdot k \cdot \frac{\lambda}{8}}}}} \\{= \quad {{{- j} \cdot \left( {r_{g} - \overset{\_}{r_{g}}} \right) \cdot ^{{- j} \cdot 4 \cdot \pi \cdot \frac{d}{\lambda}}} + {j \cdot \left( {r_{e} + \overset{\_}{r_{e}}} \right)}}} \\{= \quad {{2 \cdot {{Im}\left( r_{g} \right)} \cdot ^{{- j} \cdot 4 \cdot \pi \cdot \frac{d}{\lambda}}} + {j\quad \ldots \quad {2 \cdot {{Re}\left( r_{e} \right)}}}}}\end{matrix}$

As before, to obtain a favoured direction of propagation, we seek toobtain a pure real reflection coefficient, i.e.: $\begin{matrix}{{{2 \cdot {{Im}\left( r_{g} \right)} \cdot ^{{- j} \cdot 4 \cdot \pi \cdot \frac{d}{\lambda}}} + {j\quad \ldots \quad {2 \cdot {{Re}\left( r_{e} \right)}}}} = 0} & (3) \\{d = {{\frac{\lambda}{4 \cdot \pi} \cdot a}\quad {\sin \left( \frac{{Re}\left( r_{e} \right)}{{Im}\left( r_{g} \right)} \right)}}} & \quad \\{{with}\quad {Im}\text{:~~imaginary~~part}} & \quad \\{{{and}\quad \Gamma} = {{2 \cdot {{Im}\left( r_{g} \right)} \cdot \cos}\quad \left( {4 \cdot \pi \cdot \frac{d}{\lambda}} \right)}} & \quad \\{{\text{hence:}\quad \Gamma} = {2 \cdot {{Im}\left( r_{g} \right)} \cdot {\cos \left( {a\quad {\sin \left( \frac{{Re}\left( r_{e} \right)}{{Im}\left( r_{g} \right)} \right)}} \right)}}} & \quad \\{\Gamma = {2 \cdot {{Im}\left( r_{g} \right)} \cdot \sqrt{1 - \frac{{{Re}\left( r_{e} \right)}^{2}}{{{Im}\left( r_{g} \right)}^{2}}}}} & \quad\end{matrix}$

As in the previous example, it is possible to determine a value d suchthat it permits |Re(re)|<|Im(rg)|.

And as in the case of transducers with 4 electrodes per λ, it may bevery advantageous to make transducers in which the mesas and theetchings do not have the same width.

Examples of Transducers with 3 Electrodes Per Wavelength λ

According to another variant of the invention, it is possible to use aconventional surface wave transducer of SPUDT type, using an asymmetricnetwork of electrodes and in which the coupling performance is enhancedby virtue of the improvement in the reflection coefficients of theelectrodes used for a transducer wavelength. FIG. 7a illustrates atransducer with 3 electrodes per λ, two electrodes are λ/4 apart so asto cancel the sharp reflections of the said electrodes; specifically, awave emitted by an electrode is in phase opposition with respect to thewave reflected by the consecutive electrode separated by the distanceλ/4. The third electrode separated by a distance of 3λ/8 from theconsecutive electrode, plays the role of reflector.

One way of increasing the reflection coefficient of such a layoutconsists in increasing the thickness of the said electrodes. In general,beyond a certain value of electrode metallization, the said electrodeloses its properties and the technology becomes tricky. This is why theinvention proposes a transducer variant in which the coefficients ofreflection of the electrodes are improved by increasing the thickness ofelectrodes without increasing the thickness of metallization asillustrated in FIG. 7b. Such a technology moreover makes it possible touse a single mask for making the etching network and the electrodemetallization network.

To perfect such a structure, it is beneficial not to etch the substrateuniformly as illustrated in FIG. 7c. Specifically, by not etching at thelevel of a transduction centre, it is possible to increase thereflectivity spread so as selectively to make certain electrodes intothe reflection centre and thus lead to a decrease in transductionlength. In particular, by not etching at the level of a transductioncentre situated between an electrode referenced + and an electrodereferenced −, a surface wave at the level of this transduction centrecan be placed in phase with a wave reflected by the electrode making upa reflection centre located at 3λ/8.

What is claimed is:
 1. A surface wave transducer comprising: a substrateon which are deposited two networks of interdigital electrodes connectedto different polarities and configured to create acoustic transductioncells defined by at least two consecutive electrodes of differentpolarities; and at least one etching network separated by mesas, whereinthe superposition of the two networks of interdigital electrodes and theat least one etching network is configured to obtain a favoureddirection of propagation of acoustic waves, and wherein the two networksof interdigital electrodes are symmetric with respect to an axissituated at the centre of two consecutive electrodes of the samepolarity with respect to said axis.
 2. The surface wave transduceraccording to claim 1, wherein a central axis coincides with an etchingflank of the etching network.
 3. The surface wave transducer accordingto claim 1, further comprising a succession of at least two etchingnetworks which are asymmetric and configured to locally reverse thefavoured direction of propagation of the acoustic waves.
 4. The surfacewave transducer according to claim 1, wherein at least one etchingnetwork comprises an alternation of etchings and of mesas, a width ofthe etchings being different from a width of the mesas.
 5. The surfacewave transducer according to claim 1, wherein a distance between twoconsecutive electrodes is equal to a distance between a consecutive mesaand etching.
 6. The surface wave transducer according to claim 1,wherein a distance between consecutive electrodes is equal to a quarterof a characteristic wavelength corresponding to a central frequency ofoperation of the transducer, and the two networks of interdigitalelectrodes comprise pairs of interdigital electrodes.
 7. The surfacewave transducer according to claim 1, wherein a distance between aconsecutive mesa and etching is equal to a quarter of a characteristicwavelength.
 8. The surface wave transducer according to claim 5, whereina width of the electrodes is equal to an eighth of a characteristicwavelength.
 9. The surface wave transducer according to claim 1, whereinthe two networks of interdigital electrodes define sets of 3 electrodesper characteristic wavelength, in which sets a first electrode and asecond electrode are separated by a distance equal to a quarter of acharacteristic wavelength, and are connected to different polarities,the second electrode and a third electrode being separated by a distanceequal to three-eighths of a characteristic wavelength in such a way asto define a favoured direction of propagation of the acoustic waves, theelectrodes being positioned on the mesas of the etching network.
 10. Thesurface wave transducer according to claim 2, wherein a central axis issituated a nonzero distance from an etching flank of the etchingnetwork.
 11. The surface wave transducer according to claim 2, wherein adistance between two consecutive electrodes is equal to half acharacteristic wavelength.
 12. The surface wave transducer according toclaim 11, wherein a width of the electrodes is of the order of a quarterof a characteristic wavelength.
 13. The surface wave transduceraccording to claim 11, wherein a distance between a consecutive etchingand mesa is equal to a quarter of the characteristic wavelength, theelectrodes being positioned astride the said mesas and etchings.
 14. Thesurface wave transducer according to claim 11, wherein a distancebetween a consecutive etching and mesa is equal to the characteristicwavelength.
 15. The surface wave transducer according to claim 9,wherein the first and second electrodes of a set of 3 electrodes perwavelength are positioned on one and the same mesa.